Job postings can represent the ceiling of demand for a job in your region, but only if employers are actively advertising online
Job openings take a fairly conservative approach to demand, accounting for job growth and estimating replacement needs for workers who change careers or retire
It's likely that the true demand for a job is somewhere between job postings and openings
Job postings are released on the internet by companies hoping to attract applicants. Job openings are a measure of demand that takes into account actual growth and estimated replacement needs. Both job postings and job openings are helpful for assessing the demand for an occupation but it's important to understand the strengths and weaknesses of each.
Postings are voluntary, and therefore only represent the jobs that employers choose to advertise online. This results in certain jobs being overrepresented by job postings in relation to the actual number of positions available, while other jobs are underrepresented. However, since a posting is designed to attract applicants, it frequently contains much more detailed information about that potential job, information like desired skills, detailed job titles, and the company interested in hiring.
Because coverage can be spotty and one posting rarely equals one job opportunity, we like to think of postings as bringing useful detail, but not a true representation of demand, to the table.
Job openings are estimated using a rate of replacement per occupation developed by the BLS. When considering what workers need to be replaced, the BLS restricts its scope to "workers retiring or otherwise permanently leaving an occupation." This accounts for worker deaths and career changes in addition to retirements, but leaves out replacement needs from workers simply moving out of the region or switching jobs within an occupation.
Lightcast incorporated the BLS's new replacements methodology (known as the "Separations" method by the BLS) into its 2017.3 datarun, and future dataruns will use this methodology as well. The BLS's new methodology corrects deficiencies in the old method's ability to track the number of young workers leaving occupations and older workers entering new occupations. As a result, the improved methodology gives much higher replacement figures. Users should expect to see large increases in the number of replacements and openings compared to pre-2017.3 Lightcast dataruns.